IEEE Spectrum April, 2013 - 30

on Target

national laboratories, as well as substantial computational clout. codes are not perfect," Adams says. NIF officials say that improving
This year, Livermore's new 16-petaflops Sequoia supercomputer, those codes with results from the facility's experiments could help
stockpile caretakers understand the impact of potential changes.
which held the title of world's fastest for a few months in 2012, is
But how exactly will those results help maintain the stockpile?
to be commissioned for classified weapons-simulation work. The
Not surprisingly, scientists and officials affiliated closely with NIF
output of this research program is supposed to support the NNSA's
generally decline to give much detail, citing the need to keep the
"life extension" effort, which focuses on examining and upgrading
information confidential for national security reasons. "Once we
aging nuclear weapons to ensure that they are still safe and can be
get specific, we get classified," Adams says.
detonated reliably for another 30 years or so.
There are two ways ignition comes into play in a modern nuclear
In the absence of nuclear testing, experiments like NIF are important, says Marvin Adams, a professor of nuclear engineering at weapon. The first stage of a warhead, the primary, consists of a
hollow sphere made of plutonium or uranium that is filled with
Texas A&M and chair of the weapons science review committee at
Los Alamos National Laboratory, in New Mexico. Weapon materi- a gas made of two isotopes of hydrogen-deuterium and tritium.
als degrade, and safety and environmental regulations introduced When high explosives placed around the sphere are triggered, the
blast compresses and compacts the shell,
over the past four or five decades have made
setting off a fission chain reaction of splitit extraordinarily difficult to reconstitute
ting atoms. The process sends neutrons and
the ability to make weapons components
heat into the gas-filled center of the comexactly as they were made during the Cold
pacting shell, triggering fusion of the gas.
War, Adams says.
This fusion step, called boost, releases neuWhat's more, there are still a number of
trons that stream out through the shell, splitoutstanding questions when it comes to perting yet more atoms and releasing enough
formance. During the years of underground
heat in the form of X-rays to set off the wartesting, engineers saw anomalies and surhead's "secondary." This stage consists of a
prises in test results; in some cases, Adams
ball made of a solid mix of lightweight atoms-
says, devices that were supposed to undergo
hollow hohlRaum: NIF's ignition
typically lithium and hydrogen-that then
a critical stage of fusion didn't, and others
experiments use metallic cases, or hohlraums,
fuse to create most of the energy released
didn't release as much energy as simulalike this one to hold the material to be fused.
when a warhead explodes.
tions predicted they would. "We know our
The case is destroyed in the process.
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APR 2013

noRth AmeRicAn

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LawRence LiveRmoRe nationaL LaBoRatoRy (2)

This long position arm is used to center the target
inside the target chamber. It also serves as a
reference for alignment of the laser beams.

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Table of Contents for the Digital Edition of IEEE Spectrum April, 2013